A typical method of producing a thin plate includes rolling and forming in a rolling step. In this type of method, it is necessary before application of hot rolling to reheat a cast slab which has once cooled after casting. This process is disadvantageous from the viewpoint of energy consumption.
Recently, a direct hot rolling method has been under development. According to the direct hot rolling method, a cast slab obtained from a continuous casting machine is directly supplied to a rolling mill. Particularly, many attempts have been made to develop a continuous casting method which can produce cast slabs with rough hot rolling being omitted in the direct hot rolling process.
As a method of producing a thin cast slab, there has been proposed a method comprising continuously withdrawing a cast slab out of the mold, reducing the thickness of the cast slab using a plurality of pairs of reduction rolls while it has a liquid core, and cooling the resulting slab. This method is referred to as the liquid core reduction method.
Japanese Patent Publication No. 6-28790/1994 discloses a method of reducing the thickness of cast slabs having a liquid core with pairs of reduction rolls. According to this method, a series of spacers each having a different thickness are inserted between each pair of rolls to produce a cast slab having a target thickness by means of an arrangement in which the thickness of spacers gradually increases for downstream pairs of rolls and the roll gaps between the pair of rolls gradually decrease for downstream pairs of rolls. Cast slabs are subjected to a reduction force corresponding to roll gaps in each of which spacers are inserted. In addition, Japanese Patent Publication No. 6-28789/1994 discloses a method of carrying out casting by applying to each pair of rolls a reduction force which is increased depending on the time required until an unsolidified cast slab withdrawn from a mold reaches respective reduction rolls so that a given amount of roll reduction may be applied to the cast slab before finishing the roll reduction.